1. Field of the Invention
The present invention generally relates to an acoustic wave filter and an acoustic wave duplexer, and more particularly, to an acoustic wave filter including a ladder filter having acoustic wave resonators connected in series and parallel arms, and an acoustic wave duplexer using the acoustic wave filter.
2. Description of the Related Art
As mobile communication systems have developed, portable telephone devices and mobile information terminals have been rapidly spreading these days. For portable telephone devices, high-frequency bands such as the 800 MHz to 1.0 GHz band and the 1.5 GHz to 2.0 GHz band are used. High-frequency filters and antenna duplexers using high-frequency filters are employed in those mobile communication systems.
Examples of high-frequency filters include a ladder filter that has acoustic wave resonators connected in series and parallel arms and functions as a bandpass filter having a predetermined passband. An acoustic wave resonator that can be used in a ladder filter may be a surface acoustic wave (SAW) resonator that is small in quantity and lightweight with an excellent shape factor, or a film bulk acoustic resonator (FBAR) that exhibits excellent characteristics at high frequencies and can be made small in size.
A ladder filter as a bandpass filter is expected to achieve a large stop band attenuation. Japanese Unexamined Patent Publication No. 2004-7250 discloses a technique of increasing the stop band attenuation by connecting inductors to parallel-arm resonators in series (prior art 1). Japanese Unexamined Patent Publication No. 10-163808 discloses a technique of increasing the stop band attenuation by connecting the parallel-arm resonators to one another and connecting an attenuation pole to the parallel-arm resonators in series (prior art 2).
However, in the prior arts, it has been difficult to obtain a desired stop band attenuation in the vicinity of the passband and desired attenuations at high frequencies, simultaneously. For example, the attenuation in the vicinity of the passband is essential to attenuate a reception signal in a transmission filter of an antenna duplexer. Meanwhile, the attenuations at high frequencies are essential to attenuate the second harmonic wave and the third harmonic wave of the transmission frequency.
The attenuation characteristics of the acoustic wave filter of the prior art 1 were calculated. FIGS. 1A and 1B show the circuit structures of the acoustic wave filters used for the calculations. FIG. 1A illustrates the prior art 1, and FIG. 1B illustrates a comparative example. As shown in FIG. 1A, series-arm resonators S1, S2, S3, and S4 are connected in series, and parallel-arm resonators P1 and P2 are connected in parallel, between an input terminal Tin and an output terminal Tout in the prior art 1. With this structure, a ladder filter is formed. Inductors L01 and L02 are series-connected to the parallel-arm resonators P1 and P2, respectively. The inductors L01 and L02 are grounded in the package. The inductances of the inductors L01 and L02 are 1.0 nH and 0.3 nH, respectively, and the passband frequency (f0) of the ladder filter is 824 MHz to 849 MHz.
As shown in FIG. 1B, parallel-arm resonators P1 and P2 of a ladder filter that are the same as those shown in FIG. 1A are connected to each other on a chip, and are series-connected to a common inductor L01. The inductance of the inductor L01 is 1.0 nH, and the passband frequency (f0) is 824 MHz to 849 MHz.
FIGS. 2A and 2B show the attenuations in relation to the frequencies of the filters of the prior art 1 and the comparative example. FIG. 2A shows the attenuations in the vicinity of the passband, and FIG. 2B shows the attenuations at 0 GHz to 3 GHz. In each graph shown in FIGS. 2A and 2B, the prior art 1 is indicated by a solid line, and the comparative example is indicated by a broken line. As shown in FIG. 2B, the prior art 1 exhibits a larger attenuation than the comparative example at frequencies of approximately 1.2 GHz and higher. However, as shown in FIG. 2A, the prior art 1 exhibits a smaller attenuation than the comparative example in the vicinity of the passband of 870 MHz to 960 MHz.
Next, the attenuation characteristics of the acoustic wave filter of the prior art 2 were calculated. FIG. 3 illustrates the circuit structure of the acoustic wave filter used for the calculations. As shown in FIG. 3, parallel-arm resonators P1 and P2 of a ladder filter that are the same as those of the prior art 1 shown in FIG. 1A are connected to each other. A parallel resonance circuit formed with an inductor L01, a capacitor C3, and an inductor L3 is connected to the parallel-arm resonators P1 and P2, and the parallel resonance circuit is grounded. The inductances of the inductors L01 and L3 and the capacitance of the capacitor C3 are 1 nH, 2 nH, and 5 pF, respectively. The passband frequency (f0) is 824 MHz to 849 MHz.
FIGS. 4A and 4B show the attenuations in relation to the frequencies of the filters of the prior art 2 and the comparative example. FIG. 4A shows the attenuations in the vicinity of the passband, and FIG. 4B shows the attenuations at 0 GHz to 3 GHz. In each graph shown in FIGS. 4A and 4B, the prior art 2 is indicated by a solid line, and the comparative example is indicated by a broken line. As shown in FIG. 4A, the prior art 2 exhibits a larger attenuation than the comparative example at 865 MHz to 870 MHz, and, as shown in FIG. 4B, the prior art 2 exhibits a larger attenuation than the comparative example at frequencies of 2.1 GHz and higher. However, as shown in FIG. 4A, the prior art 2 exhibits a smaller attenuation than the comparative example in the vicinity of the passband of 870 MHz to 960 MHz.
As described above, in the prior art 1 and the prior art 2, both a desired stop band attenuation in the vicinity of the passband and a desired attenuation at high frequencies cannot be obtained at the same time. If the attenuation at high frequencies is increased, the attenuation in the vicinity of the passband decreases.